In modern times we divide the earth up into rings of lines, latitude and longitude, as wikipedia will explain.

Finding your latitude is easy, finding your longitude is a difficult process, and it was vitaly important when people started to sail large distances, the book contained lots of stories of sailors being suddenly suprised by the appearance of land - because they'd misjudged their position.

Having four ships, containing garlic, pepper, and other goods of value exceeding the total wealth of the UK, sink all at once was a major blow. Not to mention the large number of sailors who lost their lives.

There were several solutions proposed, involving steady hands and telescopes, etc, but the book mostly discusses John Harrison and his use of watches/clocks.

John Harrison was featured in Only Fools & Horses, as the designer of the watch that made Delboy & Rodney millionaires.

The idea of using a clock is that you take one with you, set to the time of your departure location. Using that clock you can compare the time to the local-time, by viewing the sun, etc. Calculating the difference between the two times allows you to see how far away, in degrees, from your port, and thus how far you've traveled.

Until harrison came along clocks weren't accurate enough to keep time. His clocks would lose a second a month, until then clocks might lose 15 minutes a day. (With more variations depending on temperture, location, and pressure. Clearly things like pendulum clocks weren't suitable for rocking ships either.)

All in all this book was a great read, there were mentions of Galilao, Newton, and similar folk who we've all heard of. There was angst, drama, deceit, and some stunning craftmanship.

Harrison was a woodworker, and he made his clocks out of wood (+brass where necessary). Choosing fast/slow-grown wood depending on purpose, and using wood that secreted oils naturally allowed him to avoid lubrication - which improved accuracy, as lubricants tend to thin/thicken when temperature/pressure change.

A lovely read, thank you very much.

In other news I received several patches for my templer static-site generator,
and this has resulted in much improvement. I've also started using Test::Exception now, and slowly updating all my perl code to use this.